Certain electronic devices require regulated inputs, either regulated voltage or regulated current, to insure they are stable and provide proper operation. Prior to the advent of the "switching regulator", the common circuits utilized were dissipative in character, being either a series or a shunt regulator in which the dissipation of power in the series or shunt element was utilized to regulate the voltage or the current supplied to that load. As may be recognized, these regulators often include power regulating transistors and function in a continuous mode thereby dissipating very large amounts of power at high load currents, especially when the input-output voltage differential is large.
The switching regulator which is utilized in the present invention has a high efficiency under virtually all input-output conditions. Furthermore, since the power transistor acting as a switch is always either in the fully cutoff or fully saturated state, the switching regulator can achieve very good regulation despite large changes in input voltage or current and thereby maintain a high efficiency over wide range of load current. Because the switching regulator circuit regulates by varying the duty cycle of the transistor switch, the switching frequency should be made very much higher than the line frequency. By doing this, the circuit elements used in the power supply may be generally small, light-weight low cost devices with relatively small power requirements. Further, because of the utilization of these components, it may be possible to drive the switching regulator with a substantially un-filtered DC, further eliminating expensive filtering elements utilized in the prior art devices.
While switching regulators have been known in the recent past, their applications have been as a voltage regulator. In these applications, a power source is connected to the switching transistor which is, in turn, in series with the load the voltage to which is to be regulated. In parallel with the load in the switching transistors, there commonly is a filter including a catch diode to provide a circulating current. The time constant of the FILTER CKT determines the base frequency at which the transistor switches. This type of voltage regulator is now beginning to find wide usage in electronic applications particularly where previous regulator devices generated large power dissipation and the application required means to dissipate the heat generated in such devices. Prior to the present invention, however, switching regulators have not been utilized in a current regulating capacity and it is here that such devices may find an extremely important application. It should be recognized that the switching voltage regulator still utilizes power dissipation in the load as a mechanism by which the voltage is regulated and the present current regulator circuit represents a marked departure from past practice. The inclusion of the switching transistor in the present circuit alleviates the need for a separate series or shunt device and the attendant power transformers and other devices including heat-dissipating units.
None of the present applications of switching regulators or the past applications of conventional current regulators known to me suggest themselves to utilization of a switching element, i.e., a switching transistor in a current regulation circuit.
One of the continuing problems of a current regulation circuit is the inherent heating of the sense component as current flows through the component varies. Such induced temperature change, compounded with changing current flow in the usual resistive regulative devices has made current regulation extremely difficult. In the present application, a special sensing resistor is used which exhibits an extremely low temperature coefficient and relatively low resistance at the same time, which device provides little additional impact on the load circuit the current to which must be regulated. It can be appreciated by one skilled in the art that any such low thermal coefficient device will enhance the operation of such a current regulating device. The inclusion of such a low coefficient device in the present current regulator further enhances its characteristic in being an extremely high efficiency regulator.
While those skilled in the art recognize there are special advantages in using a switching device for regulation of voltage, they also recognize there are substantial disadvantages in the utilization of such devices. It must be pointed out that the utilization of a switching element in a current supply source dictates that the current is initially delivered to a load circuit in a succession of "square wave" pulses. These inherently have short rise and fall times and, as may be recognized by those skilled in the art, may be seen to introduce substantial noise components to the load. When one thinks of building a current regulator, one considers the needs of a device wherein current must be regulated, e.g. constant, not a succession of distinct pulses to a high degree. They are invited not to pick noisy devices or such distinctly "pulsing" devices since the objective is to have a smooth, accurately controlled current flow to this load satisfying the demand for the special control selected for its current supply.
These problems, however, are overcome in the present invention by either being eliminated or compensated for. In the present approach, the switching regulator is caused to function over a narrow range of voltage. Further, it is provided with an inductor which operates through a catch diode to give essentially a constant current flow during the intervals when the switching regulator is in the "off" condition. Remarkably, by such inclusion, a relatively quiet device is achieved which provides very high efficiency when compared to state of the art series or shunt devices. In this manner, a widely understood device is chosen to function in a circuit totally contrary to expected utilization and, accordingly provides advantages not known in conventional systems.
These and other advantages in the present invention will be evident upon review of the subsequent description of the preferred embodiments.